tan^2α-sin^2α=tan^2α*sin^2α
sin^2a=tan^2a-tan^2a*sin^2a
=tan^2a*(1-sin^2a) (sin^2a+cos^2a=1)
=tan^2a*cos^2a cosa=sina*tana
=sin^2a
sin^2=sin^2 恒成立,所以等式成立